G: for everyone, no swearing, no sexy stuff, no drug talk, unless it's for medicine of course. basically if you wouldn't show it to a three year old, it's not a G rated panel. Try watching some little kids movies for a guideline

PG: some swearing, but kept to a minimum, can show some mild violence, but try to stay away from things like higurashi where people are being murdered left and right, can include some mild drug references, can infer some sexy stuff, but again, kept to a minimum

PG-13: can have moderate violence, some suggestive material/sexy stuff, but no nudity or hentai, can swear, but again, don't go too over the top. Can have drug references.

18+: MUST be over 18 to enter panel, WILL be enforced, can contain strong violence, nudity/hentai, (most yaoi and yuri panels fall under this category), can swear all you D@*n well please, and can contain drug references